Wind power installations of high performance are almost exclusively equipped with adjustable rotor blades. Because of that, the performance of the wind power installation may be adjusted to the wind speed, and may be limited, if necessary, in response to high wind speeds. If there is danger of overloading by strong wind, or in order to take the installation out of operation, the rotor blades are rotated into a so-called feathered pitch, in which no driving buoyancy is created on the rotor blades. In part, it is sufficient to put a rotor blade into feathered pitch, in order to brake the wind power installation. Preferably all rotor blades are rotated out of the wind.
Usually, each rotor blade is furnished with its own adjusting drive, a so-called pitch drive, which is situated in the rotor hub. An electric drive is mostly used for the pitch drive. In response to the failure of the supply voltage, however, it must be ensured that the rotor blades continue to be adjustable, and are able to be rotated into the feathered setting.
A conventional pitch drive is described in German Published Patent Application No. 103 35 575. The drive includes a three-phase a.c. motor and a frequency converter for driving the motor. The frequency converter is made up of a rectifier that is connected to a power supply system, of a link and of a controlled inverter that generates three-phase current for the motor. An emergency energy supply is connected to the link which supplies the link with energy in response to failure of the power supply system. The emergency energy supply includes an electrical energy store, for instance, accumulators or capacitors, which are connected to the link via a switch and a post-connected charging resistor. A diode is connected in parallel to the charging resistor. If the switch is closed, a charging current is able to flow from the link via the charging resistor into the energy store. In emergency operation, a current flows predominantly via the diode from the energy store into the link.
The disadvantage with the conventional rotor blade actuating drive is that the energy store of the emergency supply has to be designed for a high voltage, especially for a high charging voltage that corresponds to the link operating voltage. Electrical energy stores for high voltages are, as a rule, constructed from a large number of accumulators or capacitors that are connected one after another. The costs of the energy store rise with the number of accumulators/capacitors connected in series. At the same time, the reliability, the maintenance friendliness and the service life of the energy store decreases with the number of the elements.
An adjusting drive for the rotor blades of a wind power installation based on a DC motor is described in German Published Patent Application No. 297 22 109. The DC motor is fed from a battery. The battery is reloaded from a rectifier. The drive, including the battery, is located in the rotor, usually in the rotor hub of the wind power installation.
Conventional lead-acid or lead-gel accumulators are little suited to be positioned in the rotating rotor hub, since they have a high requirement for space and a large weight. In addition, such accumulator systems are, as a rule, not operable independently of position. In addition, lead-acid or lead-gel accumulators are difficult to monitor, since the accumulator voltage permits only an inaccurate conclusion as to the charging state. This impairs the service life, since such accumulators have to be charged more frequently than is actually necessary, for safety reasons. Generally, lead accumulators are subject to relatively high wear, and are therefore maintenance-intensive.
It is proposed in German Published Patent Application No. 100 33 029 to implement an emergency supply device for the blade adjustment drive of a wind power installation having capacitors, particularly of the type “Ultra-Cap”, as the energy store.
However, electrical capacitors are expensive and store a low quantity of electrical energy compared to batteries and accumulators. Besides that, only a small part of the stored energy is usable for drive purposes, since the voltage decreases linearly with the amount of current drawn. If several adjustment procedures of rotor blades are to be undertaken in emergency operation, without there being the possibility of meanwhile recharging the energy store, the energy stored in a capacitor will not be sufficient under certain circumstances.